This invention is concerned with plain bearings, in particular self lubricated, or dry, bearings which require low friction in the absence of a separate lubricant fluid, particularly low levels of static, or break away, friction.
Plain bearings are known in which a backing strip of steel or comparable metal has formed thereon a lining layer comprising a porous metal matrix of sintered bronze and a low friction polymer based material and incorporating fillers, hereinafter referred to as a filled polymers infiltrated in to the interstices of the sintered matrix forming a relatively thin overlying skin, or polymer layer.
It is known to infiltrate the low friction polymer based material as an aqueous xe2x80x98mushxe2x80x99 which is pressed into the intestices of the bronze matrix to solidify it. It is also known to infiltrate the polymer based material as a xe2x80x98dry paperxe2x80x99.
Such backed infiltrated sintered bronze bearings a re used not only in, dry bearing but also in oil lubricated bearings, there being similarities and differences as discussed below.
Polytetrafluoroethylene, hereafter referred to as xe2x80x98PTFExe2x80x99, has good tribological properties and is often used as or in a lining material for plain bearings; however PTFE is soft and weak and has an unacceptably high wear rate when used alone and for practicability requires the addition of fillers which reinforce the lining material against rapid wear and low load bearing strength.
In a dry bearing, wear occurs as rubbing erosion which is directly responsible for removal of the filled PTFE, the rate of removal being determined by the roughness of the surface of the body borne by the bearing and thus the friction between the body and bearing, not only the of dynamic friction that exists between the relatively moving surfaces but also the greater static or break-away friction which has effect where there is continual starting and stopping or oscillation of the body.
In an oil lubricated bearing, wear occurs principally as a result of the lubricating oil bringing about cavitation erosion of the filled PTFE.
Although the wear mechanisms differs between dry and oil-lubricated bearings, and they require different fillers, they do share a common problem in respect of how to effect reinforcement and filling of the PTFE material.
Patent specification No. WO-A-95/02772 the contents of which are incorporated by reference, describes a plain bearing of the filled PTFE-infiltrated sintered bronze type that is specifically intended as an oil lubricated bearing. The specification summarises the perceived situation in the art regarding reinforcement and filler materials that provide strength and wear resistance, namely stating that the use of xe2x80x98smoothxe2x80x99 filamentary materials such as glass and aramid fibres per se that do not adhere to PTFE should be considered as unsuitable, before concentrating on the suitability of fibrillated aramid filaments (hereafter also referred to as aramid xe2x80x98fibresxe2x80x99) and in addition thereto particulate ionic fluoride fillers that results in a lining material having improved wear against cavitation erosion.
Whereas such cavitation erosion is not a consideration in dry bearings the strengthening effect of the fibrillated aramid fibres makes such lining material useful in dry bearings in respect of load bearing capability although of only modest performance in respect of wear by rubbing against anything but the smoothest of body surfaces borne thereby.
It is an object of the present invention to provide a plain bearing that has a lining layer of filled PTFE infiltrated sintered metal matrix and dry running strength and wear resistance better than hitherto, and method of manufacturing such bearing.
According to the present invention a plain bearing comprises a metal backing, a layer of a porous matrix of sintered metal on the backing and a filled PTFE infiltrated into the interstices of the sintered layer and standing proud of the sintered layer, wherein the PTFE is filled with 1-20% by volume fibrillated aramid fibres and 10-30% by volume glass. Preferably the glass is present in the range 15 to 25% by volume, more preferably 20%. Also it is preferable the glass exists in the form of short filaments having lengths in the range 0.05 to 0.30 mm, most preferably 0.15 mm, and a diameter in the range 0.010 to 0.020 mm, more preferably 0.012 mm. Conveniently the glass is provided as milled filament E-glass, as readily available for use as reinforcement in polyester resins, and may accordingly, be coated with silane sizing agent.
The PTFE may also be filled with particulate ferric oxide to an amount 0.5 to 2.5% by volume, preferably 1.2%, to act as a pigment and, if of suitable particle. size, as an abrasive agent. The ferric oxide preferably has particle sizes less than 0.005 mm. The PTFE may also be filled with carbon, conveniently in an amorphous form such as carbon black, to an amount 0.5 to 10% by volume, to act as a pigment and as a lubricating agent. Carbon black may be present to about 10% to result in a polymer based lining having electrical conductivity. Where such conductivity in not required the carbon black is present preferably in the range 0.5 to 10% by volume, more preferably 1.6%. The presence of ferric oxide and/or carbon black is, however optional.
Preferably the total of the fillers, including the aramid fibres, approximates to 22% by volume.
It is found that when a dry bearing made in accordance with the invention is used to bear a body, typically iron or steel, that has surface asperities which give it an abrasive and effectively high friction surface, the abrasive properties of the glass and, if appropriate, the ferric oxide reduce surface asperities on the surface of the body and effect a polishing of the surface so that after a short period in operation the degree of abrasion of the lining polymer is reduced and the overall life of the bearing is prolonged. This bearing structure, and its behaviour, is particularly impressive in respect of a body, such as a suspension component of an automobile, which not only requires a significant load bearing capacity not usually expected from polymer infiltrated sintered bronze bearing but which also undergoes continuous oscillating motion, that is, one wherein twice per oscillation cycle the component surface is stationary with respect to the bearing and thus there is static friction to be overcome at the start of each motion. Thus, the abrasive action of the lining material on surface asperities of the component reduce both static and dynamic wear-inducing frictional resistance in a bearing which, by virtue of the reinforcement by the aramid fibres, has a good load bearing capacity.
In respect of the fibrillated aramid fibres and general method of manufacture the invention follows the aforementioned Patent specification No WO-A-95/02772, namely the average length of the aramid fibres used in the invention may be 0.2-1.0 mm, their diameter prior to fibrillation may be 0.012 mm-0.015 mm, and the degree of fibrillation (as measured as Canadian Freeness) may be 200 (Du Pont method TM 0894-84, Reference TAPPI-T-227M-58).
The invention also provides a method of manufacturing a plain bearing according to the invention, characterised in that the method comprises forming a mush of the constituents of the lining layer with an organic lubricant, spreading said mush over said sintered layer, and applying heat and pressure to force said mush into the interstices of said sintered layer, to solidify the mush, and to give it a smooth surface.